A conventional manufacturing process for a semiconductor element such as an LSI or VLSI formed from a micropattern uses a reduction type projection exposure apparatus for printing and forming by reduction projection a circuit pattern drawn on a mask onto a substrate coated with a photosensitive agent. With an increase in the packaging density of semiconductor elements, demands have arisen for further micropatterning. Exposure apparatuses are coping with micropatterning along with the development of a resist process.
A means for increasing the resolving power of the exposure apparatus includes a method of changing the exposure wavelength to a shorter one, and a method of increasing the numerical aperture (NA) of the projection optical system.
As for the exposure wavelength, a KrF excimer laser with an oscillation wavelength of 365-nm i-line to recently 248 nm, and an ArF excimer laser with an oscillation wavelength around 193 nm have been developed. A fluorine (F2) excimer laser with an oscillation wavelength around 157 nm is also under development.
An ArF excimer laser with a wavelength around far ultraviolet rays, particularly, 193 nm, and a fluorine (F2) excimer laser with an oscillation wavelength around 157 nm are known to have a plurality of oxygen (O2) absorption bands around their wavelength bands.
For example, a fluorine excimer laser has been applied to an exposure apparatus because of a short wavelength of 157 nm. The 157 nm wavelength falls within a wavelength region generally called a vacuum ultraviolet region. In this wavelength region, light is greatly absorbed by oxygen molecules, and hardly passes through air. Thus, the fluorine excimer laser can only be used in an environment in which the atmospheric pressure is decreased to almost vacuum and the oxygen concentration is fully decreased. The absorption coefficient of oxygen to 157 nm light is about 190 atm 1 cm 1 (e.g., “Photochemistry of Small Molecules” (Hideo Okabe, A Wiley Interscience Publication, 1978, p. 178)). This means that, when 157 nm light passes through gas at an oxygen concentration of 1% at one atmospheric pressure, the transmittance per cm is onlyT=exp(190×1 cm×0.01 atm)=0.150.
Oxygen absorbs light to generate ozone (O3), and ozone promotes absorption of light, greatly decreasing the transmittance. In addition, various products generated by ozone are deposited on the surface of an optical element, decreasing the efficiency of the optical system.
To prevent this, the oxygen concentration in the optical path is suppressed to a low level, of several ppm order or less, by a purge means using inert gas such as nitrogen in the optical path of the exposure optical system of a projection exposure apparatus using a far ultraviolet laser such as an ArF excimer laser or a fluorine (F2) excimer laser as a light source.
In such an exposure apparatus using an ArF excimer laser beam with a wavelength around far ultraviolet rays, particularly, 193 nm, or a fluorine (F2) excimer laser beam with a wavelength around 157 nm, an ArF excimer laser beam or fluorine (F2) excimer laser beam is readily absorbed by a substance. The optical path must be purged to several ppm order or less. This also applies to moisture, which must be removed to the ppm order or less.
To ensure the transmittance or stability of ultraviolet rays, the ultraviolet path of the reticle stage of an exposure apparatus or the like is purged with inert gas. As the purge method, there is proposed a method of spraying inert gas toward a photosensitive substrate. However, oxygen and moisture cannot be satisfactorily purged (see, e.g., Japanese Patent Laid-Open No. 6-260385). As another method, the whole space near a photosensitive substrate is covered with a sealing member from the lower end of a projection optical system. However, this method is not practical because it is difficult to move the stage (e.g., Japanese Patent Laid-Open No. 8-279458).
As described above, an exposure apparatus using ultraviolet rays, particularly, an ArF excimer laser beam or fluorine (F2) excimer laser beam suffers from large absorption by oxygen and moisture at the wavelength of the ArF excimer laser beam or fluorine (F2) excimer laser beam. To obtain a sufficient transmittance and stability of ultraviolet rays, the oxygen and moisture concentrations must be reduced.
From this, it is desired to develop an effective means for purging the ultraviolet path in an exposure apparatus, particularly, the vicinities of a wafer and reticle with inert gas.
The exposure apparatus is equipped with many devices such as the motors, air compressors, electric boards, and electric cables of various units. These units generate a vaporized substance and air which leaks from the air compressors. If a purge chamber is installed on the floor to purge the overall exposure apparatus including these units, the purged space is contaminated or the oxygen concentration decreases due to a substance vaporized from the units and air which leaks from the air compressors.
When the purge chamber is arranged on the exposure apparatus main body supported by a vibration isolating mechanism, vibrations of the panel of the box-like purge chamber made of a metal thin plate or the like are mixed as noise in the signal of a laser interferometer used for stage alignment on the exposure apparatus main body, adversely affecting the stage alignment precision.
In the exposure apparatus, loading/unloading of a wafer and reticle and maintenance around the wafer stage and reticle must be executed. It is difficult to partially seal these spaces.